Bacteria detect changes in oxygen concentration and swim toward an optimum oxygen level. This response, known as aerotaxis involves an oxygen receptor, a system for processing the signal from the receptor and a behavioral change effected by the processed signal. Our studies will determine the biochemical mechanism of aerotaxis as a model for sensory transduction in the oxygen chemoreceptors of the carotid body. The receptor for aerotaxis is the terminal oxidase of the electron transport system. Fluctuations in electron transport effect behavioral change by modulating the membrane protonmotive force. Transduction in bacterial chemotaxis involves three membrane-bound methyl-accepting signalling proteins (MCP). Aerotaxis involves a fourth unidentified MCP that will be identified by oxygen-stimulated 3H-methylation of the protein. The aerotaxis MCP will be isolated with the aid of gene amplification and its biochemical role in aerotaxis analyzed. The requirements for ATP in chemotaxis, including possible involvement of protein phosphorylation or cGMP, will be investigated. Other reactions involved in signal processing and tumble regulation for aerotaxis and chemotaxis will be investigated using biochemical and genetic techniques. Of particular interest is the cheZ mutation which appears to cause an inverse (repellent) oxygen response. The cheZ protein will be isolated and characterized.